1. Field of the Invention
This invention relates generally to a system for providing signal noise reduction and, more particularly, relates to a noise reduction circuit for effectively increasing the dynamic range in a transmission system through signal compression and expansion.
2. Description of the Prior Art
A noise reduction circuit is known that is comprised of a compression circuit at the transmitter for signal compression during transmission and an expansion circuit at the receiver for signal expansion during signal reception, in order to increase the effective dynamic range of the signal transmission path. This same signal compression and expansion approach has also been applied to recording and reproduction systems, in which the signal is compressed at the input to the recording device and is expanded during reproduction. Typically, these compression and expansion systems include circuits that have a controllable transfer function, in which the transfer function of the circuit is varied depending upon signal level and/or frequency.
A noise reduction circuit of this type is shown and described in U.S. Pat. No. 3,631,365, in which the compression circuit of the noise reduction circuit includes a main signal path, an auxiliary signal path, and a summing circuit for summing the signals of the two paths to produce an output signal. The auxiliary signal path is formed of a high-pass filter having a variable cut-off frequency, a control circuit for controlling this variable cut-off frequency, and an amplitude limiting circuit. The output signal from the high-pass filter is fed to the amplitude limiting circuit through an amplifier, and the output signal from the amplitude limiting circuit is fed to the summing circuit where it is added to the main path signal. The amplitude limiting circuit is comprised of reversed, parallel-connected diodes or a similar arrangement of PN junctions and has an inherent threshold level of approximately 1.2 V.sub.p-p for silicon PN junctions. Because this PN junction threshold level is large when compared to the signal levels usually involved in circuits of this kind, an amplifier must be connected in the preceding stage of the amplitude limiting circuit so that the high-pass filter output signal level can be substantially greater than the threshold level of the amplitude limiting circuit. Therefore, this amplifier is not an incidental component but is an essential element, which acts not only as a buffer amplifier for the high-pass filter but also to set the limiter level. Because the auxiliary signal path output signal, which has been amplified by this amplifier, must be added to the main signal with only a small weighting factor, additional complicated circuitry is required, which further adversely affects signal precision and stability. In addition, reverse bias or drain currents flowing in the PN junctions of the amplitude limiting circuit adversely affect the remaining circuits in the system because of the common impedance of the ground circuit.